1. Field of the Invention
The present invention relates to an adhesive for polarizing plate. The present invention further relates to a polarizing plate and manufacturing method thereof using the adhesive for polarizing plate. The polarizing plate alone or an optical film obtained by laminating the polarizing plate can constitute an image display such as a liquid crystal display, an organic EL display or PDP.
2. Description of the Related Art
In the liquid crystal displays, for example, it is indispensable to dispose polarizers on both sides of a glass substrate providing a surface of a liquid crystal panel according to an image formation scheme adopted in the display. A polarizer is generally obtained in a procedure in which a polyvinyl alcohol-based film is dyed with a dichroic material such as iodine, thereafter, the film is crosslinked with a crosslinking agent and then, mono-axially stretched to thereby form a film. Since the polarizer is manufactured by stretching, it is easy to shrink. Since a polyvinyl alcohol-based film comprises a hydrophilic polymer, the film is very easily deformed especially in a humidified condition. Since the film itself is weak in mechanical strength, there has been a problem that the film is torn. Hence, adopted is a reinforced polarizing plate manufactured in a procedure in which a transparent protective film or transparent protective films each made from triacetyl cellulose or the like are adhered to on one side or both sides of a polarizer. The polarizing plate is manufactured by adhere the transparent protective film onto a polarizer using an adhesive.
In recent years, the range of uses of liquid crystal displays has been increased and extended to cover from portable terminals to large screen home TVs, and their standards have been defined for each application. In particular, portable terminal applications, which must be portable for users, strongly demand durability. For example, polarizing plates are required to have water resistance to such an extent that their properties and form will not change even under humidified conditions such as conditions causing dew condensation.
As mentioned above, a polarizer may be reinforced in strength with a transparent protective film to form a polarizing plate before use. As an adhesive for polarizing plate used in adhesion of the polarizer and the transparent protective film to each other, an aqueous type adhesive is conventionally preferable and, for example, a polyvinyl alcohol-based adhesive obtained by mixing a crosslinking agent into a polyvinyl alcohol aqueous solution has been employed. A polyvinyl alcohol-based adhesive may cause peeling at the interface between a polarizer and a transparent protective film in a humidified environment. This is considered because a polyvinyl alcohol-based resin, which is a main component of the adhesive, is a water-soluble polymer and a possibility of dissolution of an adhesive occurs in a situation of dewing. In order to cope with the problem, a proposal has been offered of an adhesive for polarizing plate containing a polyvinyl alcohol-based resin having an acetoacetyl group, and a crosslinking agent (Patent Document 1). An aqueous adhesive for polarizing elements is also proposed which contains a polyvinyl alcohol resin, a resin having a maleic anhydride skeleton in the structure, and a crosslinking agent (Patent Document 2).
The production of polarizing plates has a problem in that knicks (knick defects) occur in the process of bonding a polarizer and a transparent protective film together with the polyvinyl alcohol-based adhesive interposed therebetween. Knicks are local concave-convex defects, which are formed at the interface between a polarizer and a transparent protective film. Against such knicks, a method of laminating a polarizer and a transparent protective film is proposed, in which the polarizer used is a product obtained by subjecting, to a calender roll treatment under specific conditions, the surface of a polyvinyl alcohol-based film with an adjusted water content (Patent Document 3). In particular, knicks are more likely to occur when a polyvinyl alcohol-based resin containing an acetoacetyl group is used as a polyvinyl alcohol-based adhesive.
To suppress the occurrence of knicks, an adhesive for a polarizing plate is proposed which includes a resin solution containing a polyvinyl alcohol-based resin, a crosslinking agent, and a metal compound colloid with an average particle size of 1 to 100 nm (Patent Document 4). Since the metal compound colloid usually has a refractive index higher than that of the polyvinyl alcohol-based resin, the adhesive layer formed using such an adhesive for a polarizing plate tends to have a high refractive index. Therefore, reflection of light is more likely to occur at each of the interface between a polarizer and the adhesive layer and the interface between a transparent protective film and the adhesive layer, which may cause a reduction in the light transmittance of a polarizing plate. In recent years, as the contrast of liquid crystal panels has been increased, there has been a stronger demand for the improvement of the optical properties of polarizing plates.
Patent Document 5 discloses a laminated flexible polarizer including a polarizing core layer and an outer layer, which are bonded together with a silicic acid salt layer, so that the laminated flexible polarizer has thermal stability and flexibility. Patent Document 6 discloses a laminated polarizer including a polarizing core layer and a cladding layer, which are bonded together with a silicate layer, so that the laminated polarizer has thermal stability and flexibility.
Unfortunately, the silicic acid salt layer or the silicate layer made from an aqueous silicate solution has low adhering strength and therefore may often cause peeling at interfaces with a polarizer and a transparent protective film.
On the other hand, a variety of compounds are known as crosslinking agents for polyvinyl alcohol-based resins. Among them, amino-formaldehyde resin, which has high reactivity with an acetoacetyl group, is advantageously used in view of water resistance.
Unfortunately, amino-formaldehyde resin, which contains formaldehyde, has raised concerns about the effect of a strong irritating odor on working environments or the safety of the residue in products. In addition, amino-formaldehyde resin, which has high reactivity with an acetoacetyl group and therefore allows crosslinking reaction to proceed easily at room temperature, also has a problem such as a reduction in workability due to an increase in viscosity during storage, and gelation at the final stage, which makes the use of it impossible, or a shortened pot life.
To solve the above problems, the use of a metal salt as a crosslinking agent has been studied. Unfortunately, excessive addition of the metal salt may increase the refractive index of the adhesive layer, so that reflection of light may be more likely to occur at each of the interface between a polarizer and the adhesive layer and the interface between a transparent protective film and the adhesive layer, which may cause a reduction in the light transmittance of a polarizing plate. In recent years, as the contrast of liquid crystal panels has been increased, there has been a stronger demand for the improvement of the optical properties of polarizing plates.